The Quetzal is the official currency of Guatemala. It is equal to 100 centavos. Notes are in denominations of Q100, 50, 20, 10, 5 and 1, and 50 centavos. Coins are in denominations of Q1, and 50, 25, 10, 5 and 1 centavos. The US Dollar is also a recognised currency in Guatemala.
American Express and Visa are the most widely accepted credit cards. ATMs are common throughout the country. If you wish to change cash or travellers cheques for local currency, it is best to take US Dollars. If you have quetzal over at the end of your stay, try and use it up or exchange it back because it is extremely tough to do so outside the country.
Currently 1 Euro is equal to 9.9 Quetzal (10/5/2012) but check here to see the latest rate.